Æthelfrith (c. 883 – c. 904/915) was an ealdorman of southern Mercia, who flourished in the last two decades of the ninth century and the first decade of the tenth century. His father is unknown. He was married to Æthelgyth, daughter of Æthelwulf; Æthelwulf is unidentified, but a possible candidate is King Ælfred the Great's brother-in-law, the ealdorman of the Gaini who died in 901. Æthelfrith was father to four ealdormen: Æthelstan Half-King (East Anglia), Ælfstan (Mercia), Æthelwald (Kent), and Eadric (Hampshire).

"In 903 it happened to Æthelfrith, dux, that all his deeds of title perished in the destruction of a fire. Therefore impelled by such a necessity, the aforementioned dux asked King Edward, also Æthelred and Æthelflæd, who then held rulership and power over the Mercian people under the aforementioned king, also all the members of the witan, that they should permit him and give authorisation for the rewriting of replacement charters for him." Among the charters reissued was one "concerning that land of ten hides, namely at Islington, which Coenwulf, the celebrated king of the Mercians, gave to his faithful comes, Beornnoth, to be free for himself and for his heirs in perpetuity after him"[1]. It would appear therefore that Æthelfrith, or possibly his, wife, could trace his family roots to the old Mercian aristocracy.

Another charter reissued in 903 was that in which Æthelwulf had granted Risborough to his daughter Æthelgyth [2]. Risborough was later the property of Ælfgifu the wife of King Eadwig, leading to suggestions that Ælfgifu might be Æthelfrith's granddaughter, however given that an ealdorman Æthelfrith begins witnessing charters early as 883 this would require that there were two successive ealdorman of the same name, for which there is no evidence.